1. Field of the Invention
The present invention relates to a communication control technique for communication between a control device and a controlled device.
2. Description of the Related Art
Wireless communication via a wireless USB using UWB (Ultra Wide Band) has recently received a great deal of attention (e.g., Wireless Ultra Serial Bus Specification 1.0).
A wireless USB can operate on the autonomous distributed Wimedia protocol and also implement centralized wireless communication using a host as a control station.
In such a wireless USB, a “device” (controlled device) is defined as a device which wirelessly communicates with a host (control device). Examples of the device are devices (“Wimedia devices”) which comply with the Wimedia protocol, and devices (“non-Wimedia devices”) which do not comply with the Wimedia protocol. The former include a self beaconing device (SBD). The latter include a directed beaconing device (DBD).
An SBD can autonomously transmit/receive a beacon complying with the Wimedia protocol. On the other hand, a DBD can transmit a beacon complying with the Wimedia protocol not autonomously but in accordance with an instruction from the host.
In the wireless USB, the devices synchronize based on a MMC (Micro-scheduled Management Command) transmitted from the host to coexist with each other.
Alternatively, a device synchronizes with the host based on a beacon transmitted from another device (e.g., SBD or DBD) synchronized with the host. For example, a Wimedia device located outside the wireless communication range of the host synchronizes with the host via a predetermined device located within that range. Such a device will be referred to as “another Wimedia device” hereinafter.
Another Wimedia device autonomously starts beacon transmission simultaneously with activation. However, the device exists outside the wireless communication range of the host and is never recognized by it directly. The device can be recognized by the host via a predetermined device (e.g., SBD or DBD) within the wireless communication range of the host and synchronize with the host based on a beacon transmitted from the predetermined device.
More specifically, the devices included in the wireless communication network synchronize with each other in the following way.
A device (e.g., host, Wimedia device (SBD), or another Wimedia device) complying with the Wimedia protocol synchronizes with the host using continuously transmitted/received beacons.
On the other hand, a non-Wimedia device (DBD) can neither autonomously transmit a beacon nor understand a received beacon, as described above. The device directly detects an MMC (Micro-scheduled Management Command) transmitted by the host, thereby ensuring synchronization of a device complying with the Wimedia protocol.
Of the devices complying with the Wimedia protocol, another Wimedia device is located outside the wireless communication range of the host, as described above, and cannot directly receive a beacon transmitted from the host. For this reason, the other Wimedia device recognizes, via a beaconing device (SBD or DBD) within the wireless communication range of the host, the contents of a beacon transmitted from the host, and synchronizes with the host.
If another Wimedia device recognizes the contents of a beacon via an SBD, it can synchronize with the host by receiving beacons continuously transmitted from the SBD.
However, a DBD does not autonomously transmit a beacon. Hence, to synchronize another Wimedia device via a DBD, the host must instruct the DBD to start beacon transmission.
More specifically, the host transmits a beacon transmission command to the DBD, thereby giving the instruction to start beacon transmission.
However, the wireless USB does not define an arrangement for determining based on the state of the wireless communication network whether to cause the DBD to transmit a beacon. That is, setting conditions to cause the DBD to start beacon transmission is left to the designer of the wireless communication network.
For this reason, the DBD may unnecessarily transmit a beacon depending on the state (device configuration or the wireless communication state of each device) in the wireless communication network.
For example, if another Wimedia device synchronizes with the host because a device has already started beacon transmission before a specific DBD starts beacon transmission, beacon transmission by the DBD may be redundant. In this case, the redundant beacon transmission for the other Wimedia device is unnecessary beacon transmission.
If such a DBD which unnecessarily transmits a beacon is included in the wireless communication network, the DBD wastes its power. Additionally, if the number of devices which transmit beacons increases, the beacon period is prolonged proportionally. Hence, a device which receives a beacon also wastes its power.